When heat treating carbon steels, such as during case hardening or neutral hardening, the steel is placed in a heated treating furnace, and endothermic gas, which is rich in carbon monoxide and carbon dioxide, is slowly circulated through the furnace to protect the steel. Because the properties of the steel depend in part upon the nature of the gas within the furnace, it is important to know and monitor the carbon potential of this endothermic gas so that the proper treating is achieved. It is well known that the carbon potential of a gas is related to the partial pressure of oxygen in the gas. Thus, oxygen sensors can be used to monitor the carbon potential of endothermic gases.
A variety of devices are presently available for measuring the partial pressure of oxygen in a gas relative to a reference gas. Some of these devices are adapted for use in determining the oxygen content of gases in a furnace or flue wall. Typical devices are described in U.S. Pat. Nos. 4,339,318, to Tanaka et al.; 4,005,001, to Pebler; 3,699,032, to Rapp; 4,115,229, to Capone; 4,098,650, to Sayles; 4,247,380, to McIntyre; 3,598,711, to Flais; 3,597,345, to Hickam; and 4,430,192, to Maeda. These devices employ an oxygen sensor having a solid electrolyte sensor element which generates an electric potential which is proportional to the differential partial pressures of oxygen on two sides of the sensor element. By sensing the oxygen concentration on one side of the sensor element relative to ambient air on the other side of the element, those skilled in the relevant art can easily determine the constituent compositions of the gas being measured. Thus, similar devices disclosed in Bulletin P-64 and Bulletin P-82 of Ametek Corporation, a division of Thermox Instruments, are capable of analyzing the carbon potentials of gases utilizing these oxygen sensors.
Most of these devices, however, utilize custom-made, solid electrolyte sensor elements of the zirconium oxide type, which are relatively large and therefore expensive. Furthermore, since the sensors are not mass-produced, their cost is often prohibitive. If the sensor element fails for any reason, it is typically returned to the factory for renewal rather than replacement, thus resulting in substantial downtime while waiting for a replacement.
In addition to the expense of the sensor elements discussed, a number of problems have been discovered when using the above-type devices in heat treating furnace applications. The devices tend to clog up with soot deposits which require the removal of the element and the cleaning of the related apparatus. This is often very difficult with the above devices and subjects the relatively delicate and expensive sensor element to the possibility of breakage.
In different contexts, it has been known that oxygen sensors having zirconium oxide elements may be used to measure the oxygen content of exhaust gases in an internal-combustion engine. These sensors are made from the same type of materials as the above-discussed sensors, but are much shorter in length so as to fit radially into an automobile exhaust. These sensors have been mass-produced for the automotive market. Typical of these sensors is the type disclosed in U.S. Pat. Nos. 3,844,920, to Burgett, et al., 4,175,019, to Murphy, and 4,184,934, to Bode, et al. A sensor of this type is mass-producible and is presently being mass-produced by General Motors. The sensor is less expensive than the previously discussed sensors. However, due to the smaller size of the zirconium oxide elements, sensors of this type require a highly symmetrical flow pattern to achieve repeatable outputs for a given gas chemistry. The sensors are thus more susceptible to variations in flow patterns and for this reason are not well adapted for use in heat treating furnaces for measuring the carbon potential of endothermic heat treating gases. It would be desirable to utilize the less expensive and mass-produced-type electrolyte oxygen sensor to measure the carbon potential of endothermic gases in heat treating furnaces. However, the disadvantages associated with this smaller type of sensor has precluded its use in the industry.
Thus, a need exists for an oxygen probe attachable to an auxiliary outlet of a heat treating furnace for measuring the carbon potential of endothermic heat treating gas within the furnace utilizing mass-produced zirconium oxide electrolyte sensors.